Nothing stands out about last Sept. 25 in Portland. It was a Tuesday, 72 degrees, no rain.

Wait a minute. No rain?

There it is: perfect conditions for riding a bike. And so it was that the date holds the record for the most bike trips across the Hawthorne Bridge on a weekday -- 8,305 -- since the city's $20,000 bike counter went up last summer.

Turns out that the week of Sept. 24 also was the busiest week at 47,080 trips and September was the busiest month with a total of 181,028 trips.

You can see the numbers yourself at a website that tracks the bike barometer's basic stats. The counter with an electronic display stands on the northwest end of the Hawthorne and flashes an updated number when bicyclists roll over sensor tubes set up on both sides of the bridge. The counter tracks bike traffic heading in both directions.

Some of the other stats (through Saturday):

The weekday average is 5,025 trips over the bridge.

The weekly average is 28,368 trips.

The monthly average is 117,018 trips.

Total trips: 936,144 over a little more than seven months

A chart on the website shows a drop in bike trips through December (fewest trips at 75,721) after the September peak, but the count has picked up in each month since.

The downturn in winter is no surprise to Roger Geller, Portland's bike coordinator.

Earlier data collected by the city shows bike ridership typically drops in the rainy, cold season, with summer the most popular time for people to head out on two wheels.

So far, Geller says, the counter is working well. The consistent numbers will help provide better data for bike planning. The barometer records trips 24-7 and 365 days a year. Before this, the city did spot bike counts on the bridge.

"A big part of it is the encouragement of more biking," Geller says. "It helps demonstrate that bicycling is a big part of the transportation network that feeds the Hawthorne Bridge."

Now for some quick facts:

When did the counter go live?

Aug. 8, 2012. The daily counts record bike trips from midnight to midnight. Who made the counter?

View full sizeThe afternoon sun on a recent day casts shadows on the Hawthorne Bridge. This view looks west and shows the sensor tubes that log the number of bikes that bump over them.Margaret Haberman/The Oregonian

How does it work?

The one here uses pneumatic tubes on both sides of the bridge that send a pulse via radio frequency to the counter. The tubes are calibrated to read the pressure of bike tires -- not pedestrians, skateboards, pet pygmy goats or anything else that might cross the bridge. A signal also is sent via modem to a server that records the cycling information with greater accuracy and in greater detail for the city.

Why doesn't the count change when I roll by it?

The sensor tubes are set up by one of the red-and-white gates on each side of the Hawthorne. That's when the number flips over. By the time you get to the counter if you're heading to downtown, you're already counted. If you happen to see the numbers change, it's because someone's behind you on a bike or someone crossed on the other side of the bridge.

Sometimes the counts on the display don't appear to be recording the eastbound traffic.

Among other things, bridge lifts sometimes cause hiccups in the signal relay, so the real-time counter may be off on a given day, but an adjustment to an antenna on the bridge fixes the problem, Geller says. The server data is unaffected.

Any difference in the number of people riding to downtown and away from downtown?

Geller checked about a month and a half of the counter data from Aug. 12 to Oct. 1 and found 50.9 percent of 336,000 bike trips were westbound and 49.1 percent were eastbound. "Given the short period I looked at and how close the numbers are, I'd call it a wash," he says.

Busiest time?

On weekdays, 5 p.m. to 6 p.m. That's when most people are heading home from work. Morning commuters typically are more spread out, Geller says -- they usually meander in from 8 a.m. to 10 a.m.

Plans for any more counters?

The city would like to put counters on the Broadway and Steel bridges, but isn't actively pursuing a private donor now, Geller says.

We're not alone.

Seattle now has a counter similar to ours on the Fremont Bridge, installed last Oct. 11. For people who want to know how the two cities stack up: Portland cyclists rule. The Fremont counts are lower than the Hawthorne's -- 1,932 daily average, 10,633 weekly average and 42,532 monthly with a total of 255,194 through Saturday.

So, what's up with the fair-weather riders?

"In the past we have looked at the correlation between weather and bicycle trips and have seen a definite correlation," Geller says in an email. "We have not looked at weather with the new barometer data, in part, because it is very time-consuming (we have to manually check the daily weather with each day's count). We will leave that to the researchers.

"The closest we've come to analysis is reported in our annual bicycle count reports, and even then it wasn't so much weather as time of year."